HackDigest v0.3.0

Linux packaging closes the family benchmark gap format-for-format, and every
binary asset now carries a per-asset .sha256 sidecar.

Added

• Linux .rpm packages for x64 and arm64
  (hackdigest-<version>-1-linux-x86_64.rpm, ...-aarch64.rpm),
  hand-rolled with rpmbuild from a generated spec that mirrors the deb
  layout (/opt/hackdigest, a hackdigest launcher, desktop entry,
  pixmaps icon) and rivet's recipe: no %dist tag and the libtinfo
  dependency exclusion that keeps the Ubuntu-built Racket runtime
  installable on Fedora. Requires names what the runtime FFI-loads
  (gtk3, webkit2gtk4.1, openssl-libs, xdg-utils)
• Linux .zip portable archives for both architectures — the same
  self-contained tree the tar.gz carries
• The AppImage now ships for arm64 too (per-arch appimagetool binary,
  same digest-verified fetch), with the launch smoke running on both
  architectures
• The signed update feed carries a Linux arm64 tar.gz entry, so arm64
  installs get in-app updates

Changed

• Every binary asset on every platform ships a per-asset .sha256
  sidecar (macOS dmg/zip, Windows msi/zip, and the full Linux set);
  SHA256SUMS continues to aggregate them